Polarization effects in scattered light are known to provide useful information about biological material. See, e.g., W. S. Bickel et al., "Application of Polarization Effects in Light Scattering: A New Biophysical Tool," Proc. Natl. Acad. Sci. USA 73, 486 (1976), where the angular distribution of components of the scattering matrix measured for scattering angles between 3.degree. and 165.degree. of polarized light were affected by suspensions of Bacillus Subtilis. The use of polarized-light scattering for biological cell differentiation was also demonstrated by Bickel et al. and by Salzman et al. See, e.g., W. S. Bickel et al., "Polarized Light Scattering from Biological Systems: A Technique for Cell Differentiation," J. Biol. Phys. 9, 53 (1981), and U.S. Pat. No. 4,884,886 for "Biological Particle Identification Apparatus", which issued to Gary C. Salzman et al. on Dec. 5, 1989.
The above-mentioned applications of polarized light require measurement of polarized irradiance over a range of forward scattering angles (between 0.degree. and 180.degree.), and are the result of single scattering events. However, there are many biomedical applications where the properties of backward-scattered light are of interest. For example, only backscattered light is available in endoscopic procedures that are used to diagnose tissues. Time-resolved measurements of the depolarization of multiple backscattered light from turbid media have been performed by Yoo and Alfano. See, e.g., K. M. Yoo et al., "Time Resolved Depolarization of Multiple Backscattered Light from Random Media," Phys. Lett. A 142, 531 (1989). In these experiments, 5-fs laser pulses, which were linearly polarized and collimated to a diameter of 5 mm, were directed onto latex-bead suspensions. The backscattered light within the beam area was collected and recorded as a function of time. It was observed that the depolarization varies with particle size and concentration and an estimate that approximately 20 scattering events are necessary to completely depolarize the light was proffered. Linearly polarized light has also been used to illuminate the skin of patients over a broad area. See, e.g., R. R. Anderson, "Polarized-Light Examination and Photography of the Skin," Archives of Dermatology 127, 1000 (1991), where viewing the skin through another linear polarizer permitted the reflectance from the skin surface (which preserves the plane of polarization) to be distinguished. The light backscattered from within the tissue, by contrast, is more likely to undergo a change in the plane of polarization or become depolarized.
Recently, Wang et al. and Jacques et al. reported azimuthal variations of intensity in the diffuse-backscattered, linearly polarized light around the light input point when viewed through a polarizing filter. See, e.g., S. L. Jacques et al., "Polarized Light Transmission Through Skin Using Video Reflectometry Toward Optical Tomography of Superficial Tissue Layers," Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems VI, R. R. Anderson, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 2671, 199 (1996). They found that these azimuthal variations vanished at approximately 2 transport mean-free-paths (mfp'=1/[.mu..sub.s (1-g)]=1/.mu..sub.s ') from the initial laser spot. For larger distances, r, an exponential decay in the light intensity is observed comparable to the case when no polarizers are used. See, e.g., T. J. Farrell et al., "A Diffusion Theory Model of Spatially Resolved, Steady-State Diffuse Reflectance for the Noninvasive Determination of Tissue Optical Properties in vivo," Medical Physics 19, 879 (1992). Dogariu and Asakura used the azimuthal intensity variations in the polarized, backscattered light for the determination of the average photon pathlength in a scattering medium. See, e.g., M. Dogariu et al., "Photon Pathlength Distribution from Polarized Backscattering in Random Media," Opt. Eng. 35, 2234 (1996).
Accordingly, it is an object of the present invention to characterize biological tissue and biological suspensions and other turbid samples by analyzing diffuse, multiply scattered, polarized, continuous-wave light which has been backscattered from the samples after impinging on a small area thereof.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.